The Wnt signal transduction pathway, known for regulating genes involved in cell proliferation, migration, self- renewal, and survival, is dysregulated in many types of cancers, particularly colorectal cancer (CRC) where activating mutations in this pathway occur in over 80% of sporadic colorectal cancers. As a result, drugs that inhibit the pathway are highly sought-after as the basis of a new generation of innovative therapeutic agents. Unfortunately, a gap exists in the ability to target the pathway with small, drug-like molecules, a gap that is the barrier to discovering drugs targeting defects in this pathway. As part of our bed-to-bedside effort, we recently reported the FDA-approved drug niclosamide inhibits Wnt/-catenin signaling via a novel mechanism involving internalization of Frizzled receptors and downregulation of Dishevelled and -catenin. Subsequent studies showed that niclosamide holds promise to treat colorectal cancer. Specifically, we demonstrated that niclosamide selectively inhibited tumor cell proliferation in colorectal cancer cell lines and freshly resected human colorectal tumors, including cells harboring mutations in APC and -catenin. Importantly, niclosamide also inhibited Wnt/-catenin signaling and colorectal tumor growth in vivo without obvious signs of toxicity in mouse xenograft models. Based on our discovery, proposed clinical trials are being developed, in which the Wnt inhibitory activity of niclosamide in polyps will be evaluated histochemically after a run-in phase prior to colonoscopy. Given the long-standing efforts and gap in biochemical targets amenable to drug discovery, the finding that a FDA-approved drug niclosamide inhibits the pathway is highly significant and offers an opportunity to develop innovative clinical agents. Nonetheless, repurposing niclosamide to treat metastatic CRC may be limited by its anthelmintic mechanism of action and suboptimal systemic bioavailability. Identification of the biochemical target and inhibitors with better potency, selectivity and pharmacokinetic properties offer the potential to improve treatment of CRC. Our work led to the hypothesis that modulating Wnt signaling by niclosamide and optimized derivatives are useful to treat CRC and that inhibition occurs by binding a specific protein target that can be exploited for drug discovery. The objective of this proposal is to identify the target and define the mechanism of niclosamide-mediated inhibition of Wnt signaling in order to inform clinical trial designs with niclosamide, and to develop more potent and selective best-in-class Wnt signaling inhibitors with appropriate PK properties for future clinical studies. The specific aims of the proposal are to: (1) To define Structure-Activity Relationships (SAR) of niclosamide driving Wnt inhibitory activity and to identify more potent and selective inhibitors of Wnt signaling; (2) To delineate the molecular mechanism underlying niclosamide-mediated inhibition of Wnt signaling and to identify the molecular target of niclosamide; and (3) To determine the tumor inhibitory effect of novel niclosamide derivatives in vivo. Funding of this proposal will enable us to identify the biological target and identify inhibiors to progress toward clinical studies to overcome a barrier in the discovery of Wnt/-catenin inhibitors. Thus accelerate the translational application of niclosamide and its derivatives to improve patient survival.